The present invention is directed toward air pollution control equipment, and more particularly toward an apparatus and method for the energy efficient recovery of volatile organic compounds (VOC) from an air stream.
Pollution of the atmosphere by VOC is a pervasive problem. Control of VOC emissions from large, discreet sources can be accomplished relatively efficiently and economically. Much more difficult to tackle has been the problem of VOC emissions from smaller discrete sources dispersed within a large facility. For example, an automobile assembly plant or other large scale manufacturing facility may engage in operations such as degreasing and painting in remote locations. In some cases, the plant operators simply vent the VOC contaminated air stream to the atmosphere without further treatment. Not only does this have a detrimental impact on the environment, it may also be a violation of state and federal air pollution laws. The prior art has tended toward pollution control apparatus associated with each remotely located source of VOC contaminated air. Some facilities attempt to capture the VOC for disposal. Others attempt to recover the VOC for reuse. Those facilities capturing the VOC for disposal are subject to more stringent emission control standards than those recovering the VOC for reuse.
Methods and apparatus for recovering VOC contaminants from a gas stream are known in the art. Representative is Grasso, U.S. Pat. No. 5,198,000. Grasso discloses subjecting a VOC laden air stream to an absorption apparatus such as a packed column using a scrubbing liquid which absorbs the VOC from the gas stream and discharges the treated gas into the atmosphere. Grasso further teaches that the VOC laden scrubbing liquid is subjected to a separation apparatus which maybe a distillation column, stripping column, evaporator, membrane stripping device or the like. The separated scrubbing liquid is recycled to the absorption apparatus and the VOC is captured for reuse or disposal.
Apparatus such as that taught by Grasso suffer from certain inherent energy inefficiencies. For example, an application such as a paint booth may require that the air in the booth be conditioned to an appropriate temperature and/or humidity for the application or curing process. If VOC contaminated air from the paint booth is exhausted to the atmosphere after the absorption process, a great deal of thermal energy can be lost. In addition, further energy input may be required to pre-condition new air for the paint booth. Energy losses are also inherent in the separation process. Many separation methods such as distillation require a large heat input. However apparatus such as that taught by Grasso do not provide for the capture and recycling of the thermal energy added to the scrubbing liquid during separation.
The present invention is directed toward overcoming one or more of the problems discussed above.
A first aspect of the present invention is an energy efficient apparatus for removing VOC from a source of a VOC contaminated gas stream. The apparatus includes a liquid absorber, such as a stripping tower, communicating with a source of a VOC contaminated gas stream. The liquid absorber associates the VOC with a scrubbing liquid. In addition, the scrubbing liquid exchanges heat and or humidity with the contaminated gas stream. The preferred scrubbing liquid is a synthetic aromatic hydrocarbon blend, which is suitable for both VOC scrubbing and use as a heat transfer fluid. Following substantial cleaning, the temperature/humidity conditioned gas is conveyed from the liquid absorber and is recycled to the source of a VOC contaminated gas stream, typically an enclosed workspace such as a paint booth. The workspace may be under positive or negative pressure. The recycling of gas to the VOC source results in significant energy savings since there is reduced need to pre-heat cool make-up or outside air to the appropriate temperature for use at the VOC source. A conduit is connected to the liquid absorber for conveying VOC laden scrubbing liquid from the liquid absorber to a separator in fluid communication with the conduit. The separator receives the VOC laden scrubbing liquid from the liquid absorber and separates the VOC from the scrubbing liquid. The apparatus may further include a conduit for recycling the scrubbing liquid from the separator to the liquid absorber. The separator is preferably a distillation column and the liquid absorber is preferably a stripping tower. A heat exchanger may be provided in operative association between the separated scrubbing liquid and the VOC laden scrubbing liquid to be received in the distillation column for transferring heat from the separated scrubbing liquid to the VOC laden scrubbing liquid before it enters the distillation column. Preheating the VOC laden scrubbing liquid prior to distillation reduces the amount of energy which must be supplied to complete the separation of the VOC from the scrubbing liquid, and increases the overall energy efficiency of the recovery system. Alternatively, the heat recovered from the separated scrubbing liquid can be used to supplement the HVAC system of the facility. Or, a portion of the heat may be transferred to the scrubbing liquid prior to its use in the absorber. If the temperature of the scrubbing liquid is adjusted to a select level by heat exchange prior to the scrubbing process, the scrubbed gas which is recycled back to the source can be conditioned to the most effective temperature for use at the VOC source.
A second aspect of the present invention is an energy efficient method for removing VOC from a gas stream contaminated with VOC. The method includes contacting the VOC contaminated gas stream with a scrubbing liquid close to the VOC containing gas stream source to associate the VOC with the scrubbing liquid. In addition, the scrubbing liquid exchanges heat with the contaminated gas stream. The preferred scrubbing liquid is a synthetic aromatic hydrocarbon blend, which is suitable for both VOC scrubbing and use as a heat transfer fluid. A hydrophobic scrubbing liquid will facilitate heat transfer without effecting the moisture content of the gas stream. The substantially cleaned and properly conditioned gas is then re-circulated back to the VOC source. The VOC laden scrubbing liquid is conveyed to a separator. There the VOC is separated from the scrubbing liquid. The separation of the VOC from the scrubbing liquid may be accomplished by distillation. If so, heat is preferably exchanged between the scrubbing liquid separated from the VOC and the VOC laden scrubbing liquid before distillation of the VOC laden scrubbing liquid. Alternatively, any heat removed from the scrubbing liquid by means of a heat exchange may be used to supplement the facility HVAC system. Or, a portion of the heat may be transferred to the scrubbing liquid prior to its use in the absorber. If the temperature of the scrubbing liquid is adjusted to a select level by heat exchange prior to the scrubbing process, the scrubbed gas which is recycled back to the source can be conditioned to the most effective temperature for use at the VOC source.
The above method and apparatus may in an alternative embodiment provide for consolidation of VOC laden scrubbing liquid from a number of remotely located sources at a single separating apparatus. This allows for economies of scale in constructing a single large separating apparatus as opposed to many discretely located small separating apparatus. This advantage can be particularly acute when the separation is conducted in a distillation column where efficient operation of the distillation column requires a continuous supply of VOC laden scrubbing liquid.
The system is also substantially closed, thereby providing capture of process air, the VOC and the scrubbing liquid for reuse. Because the method and apparatus are a recovery system, lower VOC removal efficiencies from the VOC contaminated gas streams may be permitted by regulation than those required for disposal systems, thereby making compliance with environmental regulations easier. In addition, the increased energy efficiency of the system resulting from the heat exchange strategies disclosed above can significantly reduce operating energy and material costs.